You have tried to install Admin Tools 2.4.x on Joomla! 1.5, 1.6 or 1.7. This is not possible. Admin Tools 2.4.x only supports Joomla! 2.5 as clearly stated in
our Compatibility page. You will have to use Admin Tools 2.2.10 on your site.
Please note that since September the Joomla! project itself stopped supporting Joomla! 1.5. Joomla! 1.5 is now and end of life product, not receiving any bug fixes or security updates. We strongly recommend against using Joomla! 1.5 on live sites. At some point there might be a serious security issue discovered which will go unpatched. Similarly, only old versions of third party components will run on Joomla! 1.5. It is very likely that a security vulnerability for them is discovered at some point, making them a liability for your site. Depending on the nature of the vulnerability it is possible that your server's firewall and/or your security extension (like Admin Tools Professional) may not be able to stop the attack and you'll end up with a hacked site. Even though you are probably safe using Joomla! 1.5 for a few more months, we strongly recommend starting working on an upgrade to Joomla! 2.5.
For what is worth, upgrading from Joomla! 2.5 to future versions of Joomla! has proven to be very simple. The Joomla! 1.5 to 2.5 migration appears to be the last time you need to essentially rebuild large parts of your site. The only thing you'll have to make sure in the future is having component versions which support both the old and new version of Joomla!, e.g. Joomla! 2.5 and 3.0 when upgrading from Joomla! 2.5 to 3.0.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!